This invention relates to a voice switching system for use in a teleconference system, a hand-free telephone system, and the like.
An example of such a voice switching system used conventionally in an electronic conference system is exemplified, as a first prior art, in unexamined Japanese Patent Publication No.Hei 1 245661, namely, 245661/1989. The publication paper discloses a howling compression device which compares a transmission signal level with a reception signal level so as to detect whether or not a difference between both levels exceeds a predetermined value. When the difference exceeds the predetermined value, the howling compression device either the transmission signal or the reception signal which is detected to be lower in level. On the other hand, the howling compression device prevents an echo canceller from amending on estimated impulse response, when the transmission signal level is detected to be higher than the reception signal level.
Another example of such a conventional voice switching system is exemplified, as a second prior art, in unexamined Japanese Patent Publication No.Hei 6-253001, namely, 263001/1994. The publication paper discloses a voice control circuit which prevents a voice system of a teleconference system from an erroneous operation due to a change of a transmitting input level dependent on an amount of echo suppression by an echo canceller. For this purpose, in the voice control circuit, from an aforehead section of an echo canceller, an input level of a transmission signal is detected by a transmitting input level detector while a reception signal level is detected by a receiving input level detector. A detection output of the transmitting input level detector is then compared with that of the receiving input level detector by the use of a comparator. Dependent on a result of the comparison, an amount of attenuation by a transmission signal attenuator or a reception signal attenuator is adjusted in the voice control circuit disclosed in the publication paper.
Still another example of such a conventional voice switching system is exemplified, as a third prior art, in unexamined Japanese Patent Publication No. Hei 4-22249, namely, 22249/1992. The publication paper discloses a loudspeaking telephone system which controls an amount of attenuation of a variable attenuator only by voices through a line in which an echo is cancelled. Namely, in the loudspeaking telephone system, an output of a microphone is attenuated by a primary variable attenuator, and then, an output of the primary variable attenuator is sent to a line. A voice received through the line is attenuated by a secondary variable attenuator to be supplied to a speaker. Thus, an amount of attenuation of the primary and the secondary variable attenuators are controlled by the received voice of which an echo is cancelled in the line.
However, all of a first, a second or a third prior art disclosed in each of the above-mentioned publication papers are such techniques as suppressing howling. As will later be described more in detail, a mismatch in timing is inevitably caused to occur between a signal to be attenuated and a reference signal for determining an amount of attenuation, when a certain delay exists in either the signal to be attenuated or the reference signal. As a result, an attenuation is inserted within a conversation at an inappropriate timing to deteriorate the quality of the conversation.